1. Field of the Invention
The present invention relates to a device for forming a thin layer of dry developer particles on a developer carrying member, and to a developing apparatus for performing the developing function with the thus formed thin layer, more particularly, to the thin layer forming device for forming a thin layer of non-magnetic developer particles, and to the developing apparatus for performing the developing function with the thus formed thin layer of non-magnetic developer particles.
2. Description of the Prior Art
Conventionally, various types of apparatus have been proposed and put into practice as to a dry type one-component developer apparatus. However, in any of those types, it has been very difficult to form a thin layer of one-component dry developer, so that a relatively thick layer of the developer is used. On the other hand, the recent device for improved sharpness, resolution or other qualities has necessiated the achievement of a system for forming a thin layer of one-component dry developer.
A method of forming a thin layer of one-component dry developer has been proposed in U.S. Pat. Nos. 4,386,577 and 4,387,664 and this has been put into practice. However, they describe the formation of a thin layer of a magnetic developer, not of a non-magnetic developer. The particles of a magnetic developer must each contain a magnetic material to gain a magnetic nature. This is disadvantageous since it results in poor image fixing when the developed image is fixed on a transfer material, also in poor reproducibility of color (because of the magnetic material, which is usually black, contained in the developer particle).
Therefore, there has been proposed a method wherein the developer is applied by means of a cylindrical soft brush made of, for example, beaver fur, or a method wherein the developer is applied by a doctor blade to a developer roller having a textile surface, such as velvet, for formation of non-magnetic developer thin layer. In the case where the textile brush is used with a resilient material blade, it would be possible to regulate the amount of the developer applied, but the applied toner layer is not uniform in thickness. Moreover, the blade only rubs the brush so that the developer particles are not charged, resulting in foggy images.
A method and a device wherein a thin layer of non-magnetic developer is formed with the use of magnetic particles confined by a magnetic field, are proposed in U.S. Ser. Nos. 466,574 and 527,397, both of which have been assigned to the assignee of the subject application.
It is practically desirable in using such a device that the developer does not fall and/or scatter when the developing device is handled or carried. Additionally, if the magnetic particles are not present at a proper region, the non-magnetic developer particles which have not been transferred to a latent image bearing member at a developing station may scatter from a developer supply container at an opening of the container to which the developer returns. Further, the magnetic particles can leak through the opening. It is, therefore, desirable that the magnetic particles are confined within the container at the proper region. It has further been found that, if the circulation of the magnetic particles within the container is not sufficient, the triboelectric charging to the developer is so weak so that the non-magnetic developer particles are deposited with such a small force that the resultant developed image has a foggy background.
In this type of thin layer forming device, it is required that the magnetic particles are confined surely within the container by a magnetic particle confining member. However, in the case where the distribution of the magnetic particle diameters is so broad that there are magnetic particles having a diameter smaller than that of the non-magnetic developer particles, it is possible that those small diameter magnetic particles are undesirably contained in the formed thin layer. Also, if the flowability of the developer is increased in order to extends the life of the developer, the magnetic particles are possibly not sufficiently confined and tend to leak out. If the magnetic particles are contained in the thin layer coating, they can transfer to the latent image bearing member to deteriorate the quality of the developed image, or they can damage the latent image bearing member.